The present invention relates to a hybrid marking instrument which combines the advantageous features of a typical paste-ink ball-point pen reservoir with those of a liquid-ink ball-point pen front end assembly, and to a hybrid ink composition for use in the hybrid marking instrument.
In the writing pen art, a "paste" ink is so characterized by virtue of its high viscosity, normally in excess of 8,000 centipoise (cP). A ball-point pen which uses a paste ink needs only a comparatively simple system for controlling ink flow, comprising either a capillary tube closed at one end by the ball-point assembly or a larger diameter tube, similarly closed at the point, with a viscous follower at the open end to prevent back leaking and evaporation of the ink. The flow of ink in a paste-ink ballpen is thus dependent upon the viscosity of the ink and on the clearances between the ball and its housing.
The paste-ink ballpen has at least two serious disadvantages. First, the high viscosity of the ink causes the pen to drag on the paper during writing, and the user must therefore press harder to generate a satisfactory line. Also, in order that the ink does not smudge on the paper, the rate at which ink is metered onto the paper during normal writing must be low. As a consequence, the line intensity of a paste-ink pen is usually low.
These problems can be ameliorated by using a "liquid" ink, that is, an ink composition having a viscosity that is in the range of 1.0 to 5.0 cP. In a liquid-ink ball-point pen, the low viscosity ink gives free flowing writing at low writing forces and, additionally, provides a high intensity line. For both the manufacturer and the end user, however, there are other disadvantages inherent in the liquid-ink ballpen:
(A) The liquid-ink ballpen is not as smooth writing as the paste-ink ballpen because low viscosity liquid ink does not provide adequate lubrication for the ball. This poor lubrication also contributes to a high degree of wear of the ball in the housing.
(B) The reservoir in the liquid-ink ballpen must be vented directly to the atmosphere so that air can replace the ink used. Since the liquid inks are generally water based, they are prone to evaporation, even when humectants such as polyhydric alcohols and glycol ethers are added. In the paste-ink ballpen, the ink can be protected from evaporation by the use of a viscous follower.
(C) Liquid-ink ball-point pens are prone to "feathering," or the spreading of the ink on the paper, giving a very ragged edge to the written line. Increased uses of humectants to lessen the dryout problem (B) enhances feathering.
(D) The liquid-ink ball-point pen must employ a costly ink-flow control system which incorporates a capillary reservoir or a vacuum reservoir, as explained in U.S. Pat. No. 3,951,555, the contents of which are hereby incorporated by reference. In a capillary reservoir system, the ink reservoir includes an absorber, similar in design to a cigarette filter, which retains the ink within the reservoir by capillary attraction. In a vacuum reservoir system, the ink is held in a hollow, typically tubular reservoir by the imposition of a negative pressure which results from the configuration of ink and air-control passages connecting the reservoir to the point and a surge tank, or "collector." Ink expelled from the reservoir by changes in pressure is retained in the collector by capillary attraction, i.e., vacuum and capillary reservoir systems alike rely on capillary forces to transmit the liquid ink to the ball.
(E) Both types of liquid-ink control systems are prone to uneven, intermittent ink flow when capillary flow is interrupted. Capillary reservoir systems in particular give uneven flow control. As the ink is used up during writing, the remaining ink is also maintained in smaller capillary spaces under more and more negative hydrostatic pressures. The ink does not flow as easily from these tighter capillary spaces, resulting in a gradual decline in meter rate and, hence, a less intense written line.
Writing compositions heretofore employed in ball-point pens have generally suffered from the respective drawbacks of liquid or paste inks, as summarized above. One approach to overcoming these disadvantages has been to incorporate into an ink composition a polymeric material which permits the ink to act as a liquid under some conditions and as a paste under other conditions. For example, British Patent Application No. 2,131,040 (hereinafter referred to as "Sakura") discloses such a "hybrid" aqueous ink composition, containing 0.2 to 0.45 part by weight of xanthan gum, that is used in a ball-point pen that includes, like a typical paste-ink pen, a "follower" member in the ink reservoir. The disclosed ink composition is said to posses many desirable properties, but low pen drag and high line intensity are not mentioned. British Patent Application No. 2,094,820 (hereinafter referred to as "Fisher") discloses a different hybrid ink composition, comprising a polyelectrolyte polymer and a high polarity solvent, that is used like a liquid ink in a ballpen having a sealed, pressurized ink reservoir. The disclosed pen is said to "write . . . almost as easily as unpressurized roller pens."